Design, Fabrication, and Modification of Cost-Effective Nanostructured TiO for Solar Energy Applications - Low-cost Nanomaterials: Toward Greener and More Efficient Energy Applications - page 38

Fig. 15 SEM image of F-doped TiO 2 flowers (a). UV-visible absorption spectra of C-doped
(c) and Mo, C-codoped TiO 2 (e), respectively. b The variation of 4-nitrophenol concentration by
photoelectrocatalytic reaction with flower-like TiO 2 (a) and P-25. The visible light photocatalytic
activities (d) of C-doped TiO 2 (b) treated at different temperatures in the degradation of gaseous
toluene. Photodegradation of MB (f) by photochemical reaction with Mo, C-codoped TiO 2 (e).
(Reprinted with permission from Ref. a, b [ 334 ], c, d [ 335 ], e, f [ 343 ]. Copyright American
Chemical Society)
It has been shown that nonmetal-doped TiO 2 shows a redshift of the onset and a
higher absorption in the visible light spectrum. Metal doped TiO 2 would possess
lower energy levels so that electrons and holes can be excited by low energy
photons, which also increases the absorption of visible light [ 340 ]. For example,
Mo-TiO 2 core-shell nanoparticles were prepared by the arc-discharge method and
showed enhanced photocatalytic activity under visible light, due to the Mo-doping
in (001) TiO 2 from diffusion at the shell-core interface [ 341 ], while many studies
have revealed that single doping will increase recombination sites inside the TiO 2 ,
which will therefore increase the charge recombination [ 260 ]. Conversely, it has
also been demonstrated that co-doping TiO 2 with both nonmetal anions and metal
cations can reduce the recombination sites because of the neutralization of positive
and negative charges inside TiO 2 . This can effectively improve the charge transport
efficiency and thus enhance the photocatalytic activity [ 342 ]. For example, the
Mo-C co-doped TiO 2 powders prepared by thermal oxidation of a mixture of TiC
and MoO 3 in the air have the potential of visible light harvesting (Fig. 15 e) and
effective photoexcited charge separation, and can thus exhibit higher photocatalytic
activity when compared with anatase TiO 2 (Fig. 15 f) [ 343 ].
The fabrication of semiconductor heterostructures is one of many effective
methods developed in recent years to photoexcite and separate the electro-hole
pairs. Compared to a single semiconductor, heterogeneous semiconductors are
ideal for light-harvesting devices such as photovoltaic and photoelectrochemical